Rett syndrome is a puzzling disorder affecting young females. There is no known biological marker for this disorder whose sex- specific prevalence may be twice that of phenylketonuria. Concerted laboratory and clinical investigations are essential for the elucidation of this problem. The present project forms the clinical hub of this program and includes an initial approach to therapeutic intervention. The overall goal of this project is to extand the data base with respect to this syndrome, to clarify our understanding of the individual components of the disorder, and ultimately, to define the etiologic basis. As such, this project both adds to and receives strength from the other components of the program. The specific objectives are 1) to examine the clinical spectrum of children with Rett syndrome by increasing our patient group and by expanding our clinical assessment techniques; 2) to examine the motor-behavioral components of Rett syndrome using, as a comparison group, children with autism; 3) to examine the cognitive function of children with Rett syndrome with respect to simultaneous or differential changes; 4) to measure neurotransmitter substances in cerebrospinal fluid; and 5) to initiate a program of therapeutic intervention with neuropharmacologic agents and to assess potential effects with respect to clinical, behavioral, and cognitive function. Analytical methodologies will rely heavily on careful neurological assessment, videotape recordings of performance in structured and unstructured settings, and standard neurodevelopmental and neuropsychological assessment tools. Neurotransmitter substances are assayed using accepted analytical techniques. A strategy for drug therapy has been developed specifically to examine the effect of the opiate antagonist, naltrexone. However, the scheme for evaluation is suited to assess any treatment modality. We believe that this project will lead to substantial new information regarding Rett syndromes and will aid in uncovering the basic defect.